The present invention relates to electromagnetic digitizers and, more particularly, to a shield for use in eliminating edge effects from an electromagnetic digitizer having an active area of a grid of wires surrounded by a peripheral area of connecting wires containing sensitive interconnecting circuitry comprising a pan-shaped shield of a non-ferrous metal, the shield being sized and shaped to fit over the underside of the grid of wires and surrounding area of connecting wires, the shield comprising a center section disposed over the active area of the grid wires and spaced from the active area so as to provide no interference with the operation thereof and an outer peripheral shielding lip that is disposed in close-spaced, non-contacting relationship to the areas of sensitive interconnecting circuitry contained within the connecting wires.
An electromagnetic digitizer comprises a tablet portion such as that generally indicated as 10 in FIG. 1 and a stylus (not shown) which is moved over the tablet portion 10 to provide the digitized input to a computer, or the like. The tablet portion 10 comprises a grid of horizontal and vertical wires 12 under the surface (not shown) which are connected at their periphery to connecting wires 14. By various techniques that are not important to the present invention, electromagnetic interation is created between the grid of wires 12 and the stylus. It is this electromagnetic interaction which allows the position of the stylus on the surface of the tablet portion 10 to be sensed in order to provide the digitizing input information. The digitizing activity takes place within an "active area" of the grid wires 12 as indicated by the dashed lines so labelled.
In an electromagnetic digitizer there are usually errors created at the edge of the active area which are caused by either truncation of the electromagnetic field, unbalanced sensing, or stray radiation getting into the sensing areas. In the case of stray radiation, it is sometimes possible to protect sensitive areas by shielding the areas with a metallic plate 16 as shown in FIG. 2. Usually, such prior art shield are only partially effective and still leave considerable edge effect. Typical numbers would be plus or minus 0.030 inch error reduced to plus or minus 0.010 inch error. In digitizers where a Mylar (i.e. flexible, plastic-like sheet) grid is used such as that labelled as 18 in FIG. 3, the edge effects can almost be eliminated by wrapping the sensitive areas around a metal plate 16', thus shielding the sensitive areas. Digitizers that use a large printed circuit, (PC) card with the grid wires 12 and connecting wires 14 formed on the surface thereof are normally unable to protect these sensitive areas by either of the foregoing prior art techniques, resulting in a need for shielding and/or software error correction. Software error correction is time consuming and only somewhat effective as the exact nature of the errors is neither constant nor exactly predictable.
Wherefore, it is the object of the present invention to provide a shielding technique for digitizers that use a large PC card with the grid wires and connecting wires formed on the surface thereof which can eliminate edge effect errors up to the very edge of the active area.
Other objects and benefits of the present invention will become apparent from the description that follows hereinafter when taken in connjunction with the drawing figures that accompany it.